MIMO (Multiple Input Multiple Output) precoding is an efficient method for increasing system capacity in radio communication systems that comprise an array of transmitting antennas and/or receiving antennas. MIMO precoding requires that knowledge of the channel quality between the transmitter and the receiver is available at the transmitter. In a time-division duplex (TDD) system, knowledge of the channel quality can be acquired through measurements of the reciprocal channel; in a frequency-division duplex (FDD) system, feedback from the receiver may be used. The precoding technique can be used for both single-user MIMO systems in which there is only one receiver, and multiuser MIMO systems in which there are multiple receiver antennas that share the available frequency-time resources by being spatially multiplexed.
When implementing a MIMO communications link, it is possible to impose the restriction that all transmitting antennas have the same power. Doing so is advantageous since it can help to improve the performance of power amplifiers, for example. However, applying this constraint carries a drawback that numerical methods may then be required in order to determine the precoding for each transmitting antenna. Such numerical methods can increase the computational burden on the transmitter, this being particularly true in OFDM systems where a precoder must be computed for each individual subcarrier.
One solution for reducing the computational complexity is to group neighbouring subcarriers together and compute a single precoder for them. However, such an approach can lead to interuser interference and resultant performance degradation, since the precoder will not be ideally matched to each individual subcarrier.